Project Abstract It is well known that components of the bacterial cell wall are crucial to successful host interactions for both pathogenic and beneficial microbes. While detailed mechanistic information is available on the roles of a subset of cell wall components (i.e. lipopolysaccharides (LPS) and peptidoglycan) in these associations, few other bacterial cell surface molecules have been as rigorously investigated. Bacterial hopanoids are emerging as new determinants of the efficiency of the interactions between a subset of bacteria and their eukaryotic hosts. Hopanoids are sterol-like lipids produced by diverse eukaryote-associated bacteria, including the human pathogens in the Burkolderia cepacia complex and Bacillus cereus spp. In addition to rigidifying bacterial membranes, similarly to eukaryotic sterols, hopanoids are known to confer broad stress resistance to bacteria in culture. In native infection contexts, the significance of the increase stress resistance provided by hopanoids, and whether hopanoids confer other advantages, has not been mechanistically clarified. Recent work in the Newman lab has demonstrated that a specific class of hopanoids, extended hopanoids containing an extracellular hydrocarbon tail, regulates the efficiency of the symbiotic interaction between the nitrogen-fixing bacterium Bradyrhizobium diazoefficiens with the legume host Aeschynomene afraspera. I have determined that extended hopanoids affect the progression of the symbiosis at all stages, including the initial bacterial infection of plant roots and subsequent proliferation of bacterial symbionts within host cells. I hypothesize that the role of extended hopanoids during these stages is to enhance bacterial growth under abiotic stressors, specifically the mechanical and chemical pressures provided by the host niche. In my Research Plan, I propose to characterize further the role of extended hopanoids in the B. diazoefficiens-A. afraspera symbiosis and to use this system as a model for how hopanoids facilitate persistent infections of eukaryotic hosts more generally. In my PhD I studied the nuclear actin cytoskeleton in human tissue culture models, using approaches in quantitative microscopy and biophysics, and completing my Research Plan will require me to significantly expand my skill set. I am applying for a K99 award to support the training I need to develop both the research and non-research skills necessary to achieve my long-term career goal of establishing a successful independent laboratory at an R1 institution. I hope to synthesize approaches in biophysics, quantitative microscopy, plant cultivation and microbial physiology to study microbial responses to the chemical and mechanical environments of their hosts. I believe that my training history makes me uniquely suited to occupy this under-studied research niche, which I expect to yield key principles of microbial adaptation to hosts that will be broadly applicable to many host:microbe systems.